Plants, algae and cyanobacteria need to regulate photosynthetic light harvesting in response to the constantly changing light environment. Rapid adjustments are required to maintain fitness because of a tradeoff between efficient solar energy conversion and photoprotection. Photosynthetic organisms are subjected to a large dynamic range of light intensities, which can vary rapidly due to canopy shading, passing clouds, or sunflecks, as well as on a daily or seasonal basis. To allow optimal photosynthesis at low light intensities and to avoid photo-oxidative damage due to the formation of reactive oxygen species (ROS) under excess light, photosynthetic organisms have evolved the ability to regulate light harvesting. Under excess light, photosynthetic light harvesting is regulated by nonphotochemical quenching (NPQ) mechanisms that are responsible for dissipating excess absorbed light as heat4-7. The major and most intensively investigated component of NPQ is called qE, which is turned on and off on the time scale of seconds to minutes. qE depends on acidification of the thylakoid lumen upon formation of high ΔpH across the thylakoid membrane in excess light8. In plants, this results in two important changes that facilitate qE: conformational changes of light-harvesting complex proteins by protonation and the activation of a lumen-localized violaxanthin (Vio) de-epoxidase (VDE) enzyme. VDE catalyzes the conversion of Vio to zeaxanthin (Zea) via the intermediate antheraxanthin (Anthera). Zea and Anthera (xanthophylls with a de-epoxidized 3-hydroxy β-ring end group) are the major xanthophyll pigments that are involved in qE in plants. Zea epoxidase converts Zea back to Vio in limiting light. Together, these light intensity-dependent interconversions are known as the xanthophyll cycle (FIG. 1a).
The xanthophyll cycle is ubiquitous among green algae and plants and as explained above, is necessary for the regulation of light harvesting, protection from oxidative stress, and adaptation to different light conditions1,2. VDE is the key enzyme responsible for zeaxanthin synthesis from violaxanthin under excess light.
Mutants defective in the xanthophyll cycle and qE have been identified in the unicellular green alga Chlamydomonas reinhardtii and the model plant Arabidopsis thaliana9,10. The npq1 mutants are defective in VDE activity and are unable to convert Vio to Anthera and Zea in high light (FIGS. 1a and d). Although the Arabidopsis npq1 mutant was shown to affect the VDE gene10, the molecular basis of the Chlamydomonas npq1 mutant has been mysterious, because the Chlamydomonas genome lacks an obvious ortholog of the VDE gene found in plants and other algae.